The iPhone 3GS was the first model to let users shoot video, and the iPhone 4 will even shoot in HD (720p). It's so convenient and easy that most people whip them out to shoot a few seconds of the ball game, a fun day at the park or what have you. In the thrill of an artistic moment, you might not always consider how the iPhone is held, and you realize later you shot the whole thing in portrait orientation, at odds with most of the video-playing world. Oops.
You can pop your clip into iMovie and it can widen to fill the screen's aspect, making your subject look like Shrek. Fortunately the fix is easy, and there are two methods.
The first involves iMovie '09. Once your movie is assembled, it's time to export. From the Export menu, select Export Movie. A new sheet appears. Select HD (1280x720). Yes, you'll get the black 'pillarbox' bands on either side of your movie, but it's better than than the disfigured heads.
The second method requires QuickTime Pro (QTP), though it's basically the same process. Open your video with QTP and again, export as HD.
Shooting in landscape orientation generally requires less steps to get a no-fuss video, but you're not in deep trouble if you shoot in portrait either.